When people talk about flower pot size is that trade gallons or U.S. gallons?

I'm new at this - just started my first grow. When searching around the internet (including this forum) I've seen many suggestions on what size flower pot to use. Most of them mention the size in gallons but do not mention the type of gallons. From what I've seen 5 gallons looks like the most recommended size. Is that 5 trade gallons or 5 U.S. gallons? I've been buying nursery pots which use trade gallons as a measurement so should I get the 5 gallons size or the 7 gallon size? The seven gallon size would be about 5 U.S. gallons.

5 gallon u.s. get the 7 trade